1. Field of the Invention
The present invention relates to a hypertext apparatus capable of setting up a plurality of links between nodes. More particularly, the invention relates to a hypertext apparatus for managing hypertext composed of nodes that hold information and of link information representing the relations between the nodes, the apparatus displaying the information from each node in accordance with the link information.
2. Description of the Related Art
Hypertext apparatuses were first developed as a system capable of holding information in units called nodes, of displaying character information held by the nodes, and of linking a plurality of nodes as desired. By offering such functions, the system structures various units of information and gains access to reference information.
Recent years have seen the advent of hypertext systems that handle node-held information (called node information hereinafter) composed of so-called multimedia information. Multimedia information comprises not only characters but also monochromatic and color images, sounds and motion pictures. A number of applications for systems of this kind have been proposed.
A typical conventional hypertext apparatus is introduced illustratively in "Hypertext: An Introduction and Survey," Computer, September 1987, IEEE.
Hypertext apparatuses of the above type present the concept of "links" as a way of defining the relationship between nodes that are the units in which to hold information. The information representing the links that define the node-to-node relationship for hypertext apparatuses is provided in two sets. Illustratively, node A and node B are each referenced in bidirectional fashion using the two sets of link information. Each node comprises not only the so-called node information representing the content of the node but also a plurality of link information items for controlling the node information. These nodes together with their information constitute the main part of hypertext and are stored in primary or secondary storage.
Given the link information constituted as outlined above, a plurality of nodes may form a tree structure or a network structure that allows the node information to be displayed in a structured manner. FIGS. 1 and 2 are views that illustratively describe the concepts of nodes and links for use with typical hypertext apparatuses. FIG. 1 is an overall conceptual view of a typical data constitution composed of nodes and links, and FIG. 2 is a view depicting typical link information held by a node.
In FIG. 1, reference numerals 11, 12, 13 and 14 represent a node each, and reference numerals 15, 16 and 17 stand for a link each. As illustrated, this is an example involving four nodes (node A, node B, node C, node D) among which are established links in a tree structure (link #1, link #2, link #3).
Link information is held by each node. As shown in FIG. 2, node A 11 has its link information furnished in a list format data constitution. Specifically, between a START label 20 and an END label 24 of the data constitution are first link information 21 (link #1), second link information 22 (link #2) and third link information 23 (link #3) which define the linking relations between node A 11 and the other nodes.
In other words, as depicted in FIG. 1, node A 11 has the link information designating three links (link #1, link #2, link #3) 15, 16 and 17 which define the relations between node A 11 on the one hand, and node B 12, node C 13 and node D 14 on the other. The link information of node A 11, as illustrated in FIG. 2, has pointers arranged in the list format data constitution connecting the first link information 21 (link #1), second link information 22 (link #2) and third link information 23 (link #3). The link information is stored in primary or secondary storage.
In the above example, the access to any of the nodes linked to node A 11 from the latter is executed by following successively the items of link information from the START label 20 to the END label 24 within the node 11. Each of the link information items (21-23) held in the list format data constitution designates the corresponding node which may be referenced as needed. The order in which to reference the nodes is fixedly determined by the order of the link information items connected by the list format pointers from the START label 20 to the END label 24.
In the example above, the access from node A 11 is made first to node B 12 then to node C 13 and to node D 14, in the order in which the first link information 21 (link #1), second link information 22 (link #2) and third link information 23 (link #3) are read out consecutively. If node A 11 is to be accessed conversely from node B 12, the link information (not shown) contained in the node 12 and pointing to node A 11 is utilized for bidirectional reference.
For hypertext apparatuses that employ the aforementioned data constitution, adding a new node together with its link information to the existing group of nodes simply means adding the link information to the end of the existing link information group arranged in the list format. How this takes place will now be described in more detail.
FIGS. 3 and 4 are views explaining how a new node E is added to the node A of FIG. 1. FIG. 3 is an overall conceptual view of the data constitution of a node group supplemented by a new node, and FIG. 4 is a view showing typical link information held by the node A when new link information is added. In this example, as shown in FIG. 3, the new node E 32 along with its link 33 is added to the data constitution of the node group in FIG. 1. This updates the link information of the node A, turning the node A into a node 31. As shown in FIG. 4, the link information held by the node 31 (node A) has new, fourth link information 41 attached to the end of the third link information 23, the existing last link information.
As indicated, setting an additional link to a node on the hypertext apparatus simply means adding the new link to the end of the group of link information items associated with the group of the nodes currently stored in primary or secondary storage. Heretofore, this scheme has resulted in a number of disadvantages in connection with the handling and reading of link information on the conventional hypertext apparatus.
One such disadvantage of the conventional hypertext apparatus is that the order in which to read link information is determined fixedly by the order in which the links were generated. The reading order of the link information cannot be modified as desired. Suppose that in the example of FIG. 2, the link information of the node A is supplemented by a link #4 with respect to the newly added node E. All that happens in this case is that, as shown in FIG. 4, the new link information is connected by a pointer simply to the end of the existing link information group. If the link information of the node A held in primary or secondary storage is read by use of a link information reading function such as one offered by NoteCards, all that is acquired is a list having link #1, link #2, link #3 and link #4 arranged therein in that order, i.e., the existing list simply supplemented by the new link #4 at its end. The problem here is that after a new node has been established, it is impossible to set anew the link information of that node as N-th link information that may be read out as desired by use of the link information reading function.
It follows that, on the hypertext apparatus, it is necessary to set the items of link information with their order of display properly established beforehand. Only this procedure makes it possible to create an application program that may be displayed on the screen by reading, from primary or secondary storage and within a predetermined time, the node information held by the nodes that are linked sequentially starting from a particular node.
Another disadvantage of the conventional hypertext apparatus is the absence of means for manipulating the order of link information items that define the relations between the nodes. For example, the operational function library offered by NoteCards for use with nodes and links has no means for putting into a desired order the nodes that are linked starting from a particular link, or for modifying the established order of the nodes. For this reason, it is impossible to describe or execute an operational script that would display, in a user-designated order, the nodes that are linked from a particular node.
It follows that regarding the application program mentioned above for use on the hypertext apparatus, the already established order of the links involved cannot be modified as designated by the user or by program.